Blender rendering transparent images
I tried on my Blender 2.78 and it renders this image:

Maybe it's what you want. I don't know.
I see the last node (before the composite one, I mean) is a blur.
If I delete it the image is this:

Is that what you want?

The compositing takes some time to process, and while he does in the Render on the Image Editor window is transparent. Maybe you din't wait enough to complete the compositing.